1. Field of the Invention
The present invention relates to a milling spindle of a machine tool, and more specifically, to a milling spindle of a machine tool having an improved seal structure for preventing cutting fluid within the milling spindle from leaking from a rotary joint portion to a bearing portion or a motor.
2. Description of the Related Art
FIGS. 5 through 7 show a general structure of a milling spindle of a machine tool.
FIG. 5 is a cross-sectional view showing the structure of a milling spindle, and FIG. 6 is an enlarged cross-sectional view showing the relevant portion thereof.
A milling spindle denoted as a whole by reference number 1 includes a spindle 20 supported within the housing 10 via front-side bearings 12 and 14 and a rear-side bearing 16. A drawbar 24 for drawing a tool into or out of the spindle is disposed in the spindle 20, and at the center of the drawbar 24 is formed a through coolant passage 22 for supplying cutting fluid to the tool.
A rotor 30 of a motor is attached to an outer circumference portion of the spindle 20, constituting a motor with a stator 32.
A rotation-side joint 40 is attached to a rear end portion of the drawbar 24, which constitutes a rotary joint with a joint 50 fixed to the housing 10.
The cutting fluid supplied from a passage 52 of the fixed-side joint 50 is sent via a passage 42 of the rotation-side joint 40 to the through coolant passage 22. A minute gap G1 is formed between the fixed-side joint 50 and the rotation-side joint 40, and a small amount of cutting fluid is leaked through this gap G1.
Due to the structure of the rotary joint, it is difficult to prevent such leakage of cutting fluid completely.
Therefore, a structure is adopted in which a drain hole 60 communicating a drain chamber 62 formed within the housing 10 and the exterior of the housing is provided, so as to discharge the leaked cutting fluid to the exterior of the milling spindle.
The entry of cutting fluid to the bearing 16 or the motor unit 30 and 32 damages the drive unit composed of the bearing and motor and causes serious defects, so that a labyrinth device 70 is disposed to prevent the cutting fluid from entering the drive unit.
However, as shown in FIG. 7, if for some reason a large amount of cutting fluid C1 leaks from the rotary joint portion into the drain chamber 62, the cutting fluid may not be sufficiently discharged through the drain hole 60, and a portion of the cutting fluid C1 may pass through the labyrinth device 70 and damage the bearing 16 or the motor 30 and 32.
Japanese patent application laid-open publication No. 2002-321138 (patent document 1) discloses one example of a mechanism for preventing the cutting fluid leaked from the rotary joint portion of the spindle from entering the drive unit.